Idiopathic hypoalbuminemia explained by reduced synthesis rate and an increased catabolic rate

Albumin-deficient mouse models for studying metabolism of human albumin and pharmacokinetics of albumin-based drugs

Serum albumin is the major determinant of blood colloidal osmotic pressure acting as a depot and distributor of compounds including drugs. In humans, serum albumin exhibits an unusually long half-life mainly due to protection from catabolism by neonatal Fc receptor (FcRn)-mediated recycling. These properties make albumin an attractive courier of therapeutically-active compounds. However, pharmaceutical research and development of albumin-based therapeutics has been hampered by the lack of appropriate preclinical animal models. To overcome this, we developed and describe the first mouse with a genetic deficiency in albumin and its incorporation into an existing humanized FcRn mouse model, B6.Cg-Fcgrt(tm1Dcr) Tg(FCGRT)32Dcr/DcrJ (Tg32). Albumin-deficient strains (Alb(-/-)) were created by TALEN-mediated disruption of the albumin (Alb) gene directly in fertilized oocytes derived from Tg32 mice and its non-transgenic background control, C57BL/6J (B6). The resulting Alb(-/-) strains are analbuminemic but healthy. Intravenous administration of human albumin to Tg32-Alb(-/-) mFcRn(-/-) hFcRn(Tg/Tg)) mice results in a remarkably extended human albumin serum half-life of ∼24 days, comparable to that found in humans, and in contrast to half-lives of 2.6-5.8 d observed in B6, B6-Alb(-/-) and Tg32 strains. This striking increase can be explained by the absence of competing endogenous mouse albumin and the presence of an active human FcRn. These novel albumin-deficient models provide unique tools for investigating the biology and pathobiology of serum albumin and are a more appropriate rodent surrogates for evaluating human serum albumin pharmacokinetics and albumin-based compounds.

Lactate increases hepatic secretion of VLDL-triglycerides in humans

OBJECTIVE

The pathophysiology of hypertriglyceridemia is complex hampering effective therapeutic strategies. Increased central parasympathetic nerve activity was shown to inhibit hepatic triglyceride (TG) excretion via modulation of liver stearyl-CoA desaturase (SCD)-1 activity in rodents. We evaluated the impact of 7-h lactate clamping on VLDL-TG homeostasis in humans.

METHODS

Eight normolipidemic, male subjects were subjected to a continuous infusion of l-lactate (target concentration 3 mmol/L) or saline for 7 h in random order on two separate occasions. TG kinetics in very low density lipoproteins (VLDL1 and 2) were measured after a bolus injection of [1,1,2,3,3]-(2)H5-glycerol. Palmitic acid (16:0) and palmitoleic acid (16:1) in VLDL1 and VLDL2 were measured as a reflection of liver SCD1 activity.

RESULTS

Plasma TG levels changed by 0.16 ± 0.09 mmol/L during lactate vs -0.15 ± 0.08 mmol/L during saline (P < 0.05). VLDL1 16:1/16:0 ratio increased to 1.2 ± 0.7 during lactate versus a decrease during saline by -1.5 ± 0.6 (p = 0.01). During lactate VLDL1-TG excretion was higher compared to saline (1604 [827-2870] versus 1285 [505-2155] μmol glycerol; p < 0.05), trending toward higher VLDL1-TG pool sizes during lactate (28%; p = 0.07 versus saline).

CONCLUSIONS

In normolipidemic men, 7-h l-lactate clamp increases, rather than decreases SCD1 activity and hepatic TG secretion leading to elevated plasma TG levels. These conflicting data between human and rodents on central regulation of hepatic TG excretion illustrate that experimental findings on the role of the central nervous system in lipid metabolism should be interpreted with caution.

Albumin turnover: experimental approach and its application in health and renal diseases

Plasma albumin is an important protein in the human body and is responsible for transport and binding of many molecules. Furthermore, it is involved in mediating blood volume and colloid osmotic pressure (COP). As hypoalbuminemia occurs, as is the case in a number of clinical disorders, adaptation mechanisms may be involved. Serum albumin concentration is the net result of physiological processes like synthesis and catabolism. Measurement of one of these processes can provide therefore a more dynamic insight into the adaptation mechanism of albumin metabolism in relation to an underlying disease than would be obtained by changes in albumin concentration alone. This review highlights several studies over the past years that have contributed to knowledge of albumin metabolism. A short introduction is given for synthesis, formation and catabolism of albumin, after which an overview is given on how to measure albumin turnover including a general approach. Finally, albumin metabolism focused on patients with renal diseases will be discussed.

Processing of protein glycation, oxidation and nitrosation adducts in the liver and the effect of cirrhosis

BACKGROUND/AIMS

Plasma proteins are modified non-enzymatically in vivo by glycation, oxidation and nitrosation processes. Hepatic extraction of albumin glycated in vitro was reported but it is not clear if plasma proteins glycated in vivo also undergo hepatic extraction. We investigated the hepatic extraction of glycated, oxidised and nitrosated proteins in vivo.

METHODS

Protein glycation, oxidation and nitrosation marker residues and free adducts were determined in portal, hepatic and peripheral venous blood plasma of cirrhotic patients and hepatic and peripheral venous blood plasma (as a surrogate of portal venous blood) of control subjects by liquid chromatography-mass spectrometry.

RESULTS

There was no evidence for extraction of glycated, oxidised or nitrosated proteins or related free adducts by the liver in control subjects. There was limited extraction of methylglyoxal-modified proteins in cirrhotic patients and twofold increases in the concentrations of fructosyl-lysine and advanced glycation endproduct residues of plasma protein, with respect to controls. Remarkably, glyoxal-derived hydroimidazolone free adduct was increased 14-16-fold probably as a consequence of hepatic lipid peroxidation.

CONCLUSIONS

We found no evidence for hepatic extraction of glycated, oxidised and nitrosated proteins or related free adducts in subjects with normal liver function and limited extraction of methylglyoxal-modified protein in cirrhotic subjects.

Endogenous cholesterol synthesis is associated with VLDL-2 apoB-100 production in healthy humans

Subjects with high plasma cholesterol levels exhibit a high production of VLDL apolipoprotein B-100 (apoB-100), suggesting that cholesterol is a mediator for VLDL production. The objective of the study was to examine whether endogenous cholesterol synthesis, reflected by the lathosterol-cholesterol ratio (L-C ratio), affects the secretory rates of different VLDL subfractions. Ten healthy subjects were studied after overnight fasting. During a 10 h primed, constant infusion of 13C-valine (15 micromol/kg/h), enrichment was determined in apoB-100 from ultracentrifugally isolated VLDL-1 and VLDL-2 by gas chromatography mass spectrometry. The synthesis rates of VLDL-1 apoB-100 and VLDL-2 apoB-100, catabolism, and transfer were estimated by compartmental analysis. Mean VLDL-1 apoB-100 pool size was 90 +/- 15 mg, and mean VLDL-2 apoB-100 pool size was 111 +/- 14 mg. Absolute synthesis rate of VLDL-1 apoB-100 was 649 +/- 127 mg/day and 353 +/- 59 mg/day for VLDL-2 apoB-100. There was a strong association between the absolute synthesis rate of VLDL-2 apoB-100 and L-C ratio (r 2 = 0.61, P < 0.01). In contrast, no correlation was observed between L-C ratio and absolute synthesis rate of VLDL-1 apoB-100 (r 2 = 0.302, P = 0.09). In conclusion, these data provide additional support for an independent regulation of VLDL-1 apoB-100 and VLDL-2 apoB-100 production. Endogenous cholesterol synthesis is correlated only with the VLDL-2 apoB-100 production.